Field
This disclosure relates to radio frequency filters using surface acoustic wave (SAW) resonators, and specifically to filters for use in communications equipment.
Description of the Related Art
A radio frequency (RF) filter is a two-terminal device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low insertion loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is less than a defined value such as one dB, two dB, or three dB. A “stop-band” may be defined as a frequency range where the insertion loss of a filter is greater than a defined value such as twenty dB, twenty-five dB, forty dB, or greater depending on application.
RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.
RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between such performance parameters as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.
Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.
Surface acoustic wave (SAW) resonators are used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. A duplexer is a radio frequency filter device that allows simultaneous transmission in a first frequency band and reception in a second frequency band (different from the first frequency band) using a common antenna. A multiplexer is a radio frequency filter with more than two input or output ports with multiple pass-bands. A triplexer is a four-port multiplexer with three pass-bands.
As shown in FIG. 1, a typical SAW resonator 100 is formed by thin film conductor patterns formed on a surface of a substrate 105 made of a piezoelectric material such as quartz, lithium niobate, lithium tantalate, or lanthanum gallium silicate. The substrate 105 is commonly a single-crystal slab of the piezoelectric material, or a composite substrate including a thin single-crystal wafer of the piezoelectric material bonded to another material such as silicon, sapphire, or quartz. A composite substrate is commonly used to provide a thermal expansion coefficient different from the thermal expansion coefficient of the single-crystal piezoelectric material alone. A first inter-digital transducer (IDT) 110 includes a plurality of parallel conductors. A radio frequency or microwave signal applied to the first IDT 110 via an input terminal IN generates an acoustic wave on the surface of the substrate 105. As shown in FIG. 1, the surface acoustic wave will propagate in the left-right direction. A second IDT 120 converts the acoustic wave back into a radio frequency or microwave signal at an output terminal OUT. The conductors of the second IDT 120 are interleaved with the conductors of the first IDT 110 as shown. In other typical SAW resonator configurations (not shown), the conductors forming the second IDT are disposed on the surface of the substrate 105 adjacent to, or separated from, the conductors forming the first IDT. Also, extra fingers (commonly called “dummy” fingers) are sometimes formed opposite to the ends of the IDT fingers and connected to the IN and OUT bus bars of the first and second IDTs 110 and 120. Grating reflectors 130, 135 are disposed on the substrate to confine most of the energy of the acoustic waves to the area of the substrate occupied by the first and second IDTs 110, 120. The grating reflectors 130, 135 float or are connected to either the IN terminal or the OUT terminal. In general, the SAW resonator 100 is bi-directional, and the IN and OUT terminal designations may be transposed.
The electro-acoustic coupling between the first IDT 110 and the second IDT 120 is highly frequency-dependent. The basic behavior of acoustic resonators (SAW, bulk acoustic wave, film bulk acoustic wave, etc.) is commonly described using the Butterworth Van Dyke (BVD) circuit model as shown in FIG. 2A. The BVD circuit model consists of a motional arm and a static arm. The motional arm includes a motional inductance Lm, a motional capacitance Cm, and a resistance Rm. The static arm includes a static capacitance C0 and a resistance R0. While the BVD model does not fully describe the behavior of an acoustic resonator, it does a good job of modeling the two primary resonances that are used to design band-pass filters, duplexers, and multiplexers (multiplexers are filters with more than 2 input or output ports with multiple pass-bands).
The first primary resonance of the BVD model is the motional resonance caused by the series combination of the motional inductance Lm and the motional capacitance Cm. The second primary resonance of the BVD model is the anti-resonance caused by the combination of the motional inductance Lm, the motional capacitance Cm, and the static capacitance C0. In a lossless resonator (Rm=R0=0), the frequency Fr of the motional resonance is given by
                              F          r                =                  1                      2            ⁢            π            ⁢                                                            L                  m                                ⁢                                  C                  m                                                                                        (        1        )            The frequency Fa of the anti-resonance is given by
                              F          a                =                              F            r                    ⁢                                    1              +                              1                γ                                                                        (        2        )            where γ=C0/Cm is a characteristic of the substrate upon which the SAW resonator is fabricated. γ is dependent on both the material and the orientation of the crystalline axes of the substrate, as well as the physical design of the IDTs.
The frequencies of the motional resonance and the anti-resonance are determined primarily by the pitch and orientation of the interdigitated conductors, the choice of substrate material, and the crystallographic orientation of the substrate material.
FIG. 2B is a plot of the admittance of a theoretical lossless acoustic resonator. The admittance exhibits a motional resonance 212 where the admittance of the resonator approaches infinity, and an anti-resonance 214 where the admittance of the resonator approaches zero. In over-simplified terms, the lossless acoustic resonator can be considered a short circuit at the frequency of the motional resonance 212 and an open circuit at the frequency of the anti-resonance 214. The frequencies of the motional resonance 212 and the anti-resonance 214 are representative, and a resonator may be designed for other frequencies.
Cellular telephones operate in various bands defined by industry or governmental standards. For example, the 3GPP LTE (Third Generation Partnership Project Long Term Evolution) standard defines 48 different bands over a frequency range of about 450 MHz to greater than 5000 MHz. Each of these bands consists of a frequency range or a pair of disjoint frequency ranges used for cellular telephone communications. For example, Band 12, which is used in the United States and Canada, employs the frequency range from 699 MHz to 716 MHz for communications from the cellular device to the cellular network and the frequency range from 729 MHz to 746 MHz for communications from the network to the device. Band 40, used in several countries in Asia, employs the frequency range from 2300 MHz to 2400 MHz for communications in both directions. All of bands defined by the 3GPP LTE standard are not currently in use, and only one or a few bands are typically used in any particular country. Further, different cellular service providers in a given country may each have frequency allocations within one or multiple bands.
Antenna diversity uses two or more antennas to improve the quality and reliability of a wireless link. In urban and indoor environments, a transmitted signal may be reflected along multiple paths before finally being received. Each of these reflections introduces a phase shift or time delay Such that signals reflected along differ paths may destructively interfere with one another at the receiving antenna. This situation, commonly called “multipath fading,” can result in communications drop-outs.
The use of multiple, physically separated, antennas on a communications device is an effective technique for mitigating multipath fading. Each antenna will experience a different interference environment. If one antenna is subject to severe multipath fading, it is likely that a second antenna disposed a short distance from the first antenna will have a sufficient signal. Current communications devices, such as cellular telephones, typically include a second antenna and a second receiver chain to improve call reliability. The second antenna and receiver chain are commonly referred to as a “diversity receiver.”
In order to increase data rate, a technique called carrier aggregation may also be used. Carrier aggregation uses multiple bands simultaneously for the transmission and reception of data, thereby increasing the data throughput rate. When receiving data transmitted using carrier aggregation, it may be advantageous for the receiver to include a single filter with multiple pass-bands matching the multiple bands being transmitting.
Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.